Question

Paramedics bring a 59-year-old woman to the emergency department after she reported severe dizziness and near-syncope at home. For the past two weeks, she has experienced worsening fatigue and an inability to lie flat without profound breathlessness. She has a history of left-sided breast cancer treated with lumpectomy and radiation therapy eight years ago, as well as a 30-pack-year smoking history. Current vital signs show a temperature of $37.1^\circ\text{C}$ ($98.8^\circ\text{F}$), blood pressure of $86/52\text{ mm Hg}$ (which decreases to $72/45\text{ mm Hg}$ upon inspiration), heart rate of $122\text{/min}$, and respiratory rate of $26\text{/min}$. Arterial blood gas analysis on room air shows a pH of $7.50$, PaCO $_2$ of $28\text{ mm Hg}$, and PaO $_2$ of $88\text{ mm Hg}$. Physical examination reveals distended neck veins while sitting upright. Pulmonary auscultation shows clear breath sounds bilaterally. Cardiac auscultation reveals muffled heart sounds and no discernible murmurs. Extremities are cool to the touch without pitting edema. A 12-lead electrocardiogram demonstrates sinus tachycardia with beat-to-beat variation in the QRS complex amplitude. Which of the following is the most appropriate next step in the management of this patient?